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A common murre is washed at the Southern California Center of International Bird Rescue. The bird was most likely oiled from natural seeps off the coast of Santa Barbara. Oiled birds go through multiple washes after they are healthy enough for a bath. ( International Bird Rescue -- Contributed)

Julie Skoglund, of the Southern California Center of International Bird Rescue, examines a brown pelican covered in oil, which most likely came from natural seeps off the coast of Santa Barbara. ( International Bird Rescue -- Contributed)

SANTA CRUZ — The television images after a catastrophic oil spill, such as the one caused by the container ship Cosco Busan’s 2007 collision with the Bay Bridge, are often stark and heartbreaking — thousands of birds covered in oily tar struggling for their lives.

But marine birds smeared with oil continuously wash up on California beaches, and not just after large accidents. The culprit: nature.

Oil from natural seeps accounts for 9 of 10 oiled birds found along California’s coast in the average year, according to researchers at Santa Cruz’s Marine Wildlife Veterinary Care and Research Center. Researchers counted oiled birds washing up on California’s coast and sent their greased feathers for “oil fingerprinting” to identify the origination of the oil.

Before working on the study, “I didn’t know much about these natural oil seeps in California,” acknowledged Laird Henkel, the center’s director. “We are guessing that more than 1,000 seabirds are oiled each year by this natural source of oil.”

Similar to the La Brea tar pits in Los Angeles, natural seeps are cracks in the ground where oil oozes out. Worldwide, nearly 200 million gallons of oil pour into marine ecosystems annually from such seeps, according to the National Research Council. That amount is half the crude oil released into the ocean each year. Humans are responsible for the other half through discharge from ships, oil operations, pipelines, spills and extraction.

Once the oil rises to the surface, the birds come in contact with it.

“Most of the oiling occurs around the belly, called the bathtub ring,” said Hannah Nevins, a seabird biologist with the American Bird Conservancy. The birds then rub it onto their wings and, if they try to clean themselves, smear it onto their faces and beaks.

Covered in oil, the birds risk hypothermia when they dive for food; they can die from starvation or the cold.

“It’s like if you were skiing and had a hole in your down jacket,” Nevins said. “If their feathers get all gummed up, it messes up their waterproofing.”

The recently published study by the Marine Wildlife Veterinary Care and Research Center took about a decade to complete.

First, researchers asked two long-term bird monitoring groups for data on the number of birds found oiled on beaches each year.

Volunteers from Beach COMBERS, a “citizen scientist” group based in Moss Landing, scoured Central Coast beaches monthly for dead birds. At the same time, the Oiled Wildlife Care Network at UC Davis, which tracks the number of oiled birds found by the public along California’s coast, offered its bird counts to the team. The data, collected from 2005 to 2010, helped the scientists look for patterns to the oiling of marine birds. International Bird Rescue in Fairfield cared for and cleaned oiled birds found alive.

Although the number of oiled birds tallied was relatively small, diving birds such as grebes (41 percent) and murres (28 percent) were the most likely victims.

For species with large, healthy populations, the deaths from oiling will not have a large impact, but for endangered species such as the marbled murrelet, a loss of 100 birds every year could be significant, Nevins said.

The number of oiled birds peaked near the largest California seeps, located in the Santa Barbara Channel. The seeps come from the Monterey Formation — the sedimentary geological formation under much of California’s coastline.

The study also showed that more oiled birds landed on beaches during the winter. The scientists theorized that big winter storms scrape tar away from otherwise blocked seeps.

During their bird counts, the researchers gathered oiled feathers from the bedraggled birds. They sent the gooey feathers to chemistry labs for oil fingerprinting to track the oil back to its origin.

Oil is constructed of complex, Tinkertoy-style rings of carbons and hydrogens. And these patterns differ slightly, depending on the oil’s source, explained Thomas Lorenson, a U.S. Geological Survey geologist in Santa Cruz who was not involved in the study. Just like crime scene investigators match the ridges of a fingerprint at a crime scene to a suspect’s fingerprint, petroleum chemists can match oil from a feather to oil from a source.

All natural oil found on the feathers came from the Monterey Formation, and most started in the Santa Barbara Channel. Researchers also recently matched a set of samples taken from feathers, tar balls and sea otter fur to a recently discovered seep off the San Luis Obispo coast.

Oil degrades in water; bacteria nibble on the digestible bits. Seep oil, exposed to water for an extended period of time, is highly degraded compared to oil discharged directly from oil production. “You use the fingerprinting to tell if the oil is from the Monterey Formation, and you use degradation to say there is a great likelihood that it is seep oil,” Lorenson said.

Of the 11 percent of “oiling events” linked to human-caused oil sources, bilge cleaning — the dumping of oily ship water at sea — accounts for about half, Henkel said. Research shows human-caused oil pollution has declined markedly in California, most likely a result of increased regulations limiting bilge cleaning.

The study also showed that cleanup efforts on previously leaky shipwrecks were successful. Oil from the S.S. Jacob Luckenbach, 17 miles southwest of San Francisco, and the S.S. Palo Alto at Seacliff State Beach in Aptos, was cleaned up in 2002 and 2006, respectively.

Oil from the two ships accounted for 6 percent of oiled birds collected along the Central Coast, the study found. Oil from both sources has since disappeared.

“We are not aware of any other leaking shipwrecks, but that’s one of the advantages of doing the oil fingerprinting,” Henkel said. “If we find a bunch of samples that don’t fit the Monterey Formation oil, then we could potentially look for the source of that oiling.”

By identifying both seasonal and geographic patterns of different sources of oil, the scientists can be on the lookout for anomalies, specifically human-caused oil sources that need their immediate attention.

“If we see something abnormal,” Henkel said, “we can be more suspicious.”